Method and apparatus for producing electromagnetic surveillance fields

ABSTRACT

Methods and apparatus for producing alternating electromagnetic fields for use in theft detection and surveillance systems include individual series-type electrical drive circuits for separately energizing each of the field-generating coil windings with maximum efficiency and control, and transformer-type coupling between the different field-generating coil drive circuits to effect balanced and equalized current flow in the different coil windings and thereby enhance both drive efficiency and field uniformity. Additionally, a current-sense winding provides for monitoring of field coil current flow, and for feedback control of the magnitude and/or frequency of the coil excitation drive. In a preferred embodiment, the fields are generated by at least two coils on each opposite side of the field, and the coils are separately energized through separate but commonly-coupled drive circuits.

TECHNICAL FIELD

This invention relates generally to electromagnetic field generation,and more particularly to producing, maintaining, and cyclically changingalternating electromagnetic fields of predetermined strength andparticular resultant flux direction. Still more particularly, theinvention relates to the utilization of such fields in electromagneticsurveillance systems of the general type in which an alternatingelectromagnetic field is maintained within and across an egress orentrance passage for detecting the presence within the passage ofunauthorized articles or objects, e.g., merchandise, in which is hiddena marker or tag, preferably in the nature of a thin, narrow strip ofPermalloy or other such material of extremely high permeability.Accordingly, the invention relates to the nature and physical structureand arrangement of the field-generating coils for such a surveillancesystem, and also to novel and desirable systems and circuitry fordriving such coils while maintaining them in balance with one anotherthrough a number of different and varying phase relationships.

BACKGROUND OF THE INVENTION

As a general matter, the underlying concepts involved in electromagneticfield surveillance systems have been proposed heretofore by a number ofdifferent persons over a considerable span of time, dating back to atleast about 1970 or before, at about which time the early work of Dr.Edward Fearon began to be published and seen in patents, etc. Actually,the work of Fearon was itself built upon technological phenomenapublished as long ago as 1934, by the French citizen P. A. Picard, towhom French Patent No. 763,681 was issued in 1934, such patentdescribing the perturbation effects produced by Permalloy and other suchlow-coercivity materials upon an alternating electromagnetic field.Based upon these early investigations of Picard, a number of personshave heretofore proposed the use of the technological phenomena involvedfor surveillance purposes, to detect attempted surreptitious movement ofobjects and articles past a point of egress or entrance if such objectsor articles have hidden upon them a "tag" or "marker" of Permalloy orthe like.

Much effort has been spent heretofore to refine and improve the basicsystems proposed by early workers such as Fearon, for example, whohimself is named in a number of prior patents for such improvements orenhancements. Generally fpeaking, most of the prior work has been donewith the objective in mind of either improving the nature of thedetection tag or marker device or to improve the detection andsignal-analysis circuitry, primarily for the purpose of increasing thesensitivity or selectivity of the system so as to avoid erroneous alarmsignals while at the same time missing detection of as few tags ormarkers as possible. In the latter connection, reference is made topreviously-filed co-pending U.S. patent applications Ser. Nos. 358,299,now U.S. Pat. No. 4,535,323 and 358,383, now U.S. Pat. No. 4,524,350 bythe present inventors and/or co-workers, which are commonly ownedherewith.

In the aforementioned prior work, little has been indicated as torefinements and improvements in the interrogation field-generatingapparatus and methodology, apart from describing generally the largephysical size of the field-generating inductance coils themselves andnoting that the same should be made part of a resonant LC circuit whichhas seemingly always been referred to as a parallel-connected or "tank"circuit, the resonant frequency of the tank circuit being rathergenerally selected, typically from the standpoint of merely producingthe generally-desired interrogation field frequency. Apart from this,one of the few known publications or disclosures referring to particularmethods or techniques involved in field generation or fieldcharacteristics is a prior U.S. Patent to Richardson, U.S. Pat. No.4,300,183, which refers to the technique of alternately changing thephase relationships of the currents used to drive the resonant LC tankcircuits, so that the resultant direction of the flux in theinterrogation field was made to alternate in direction, therebyincreasing the likelihood of detecting marker strips or the like whichwere physically oriented in varying directions, some of which mightproduce a relatively slight or negligible change in the interrogationfield and thus obscure or preclude detection. In actuality, such phasealternation had already been known and utilized prior to that time, butgenerally speaking the concept is a valid one and as stated representsone of the few improvements made in the area of field generation orfield characteristics over the many years in which the interrogationsystems generally have been considered and/or used.

THE PRESENT INVENTION

The present invention provides a number of improvements in apparatus andmethodology addressed to generating and maintaining oscillatingelectromagnetic interrogation fields. Generally speaking, theimprovements of the present invention provide the very advantageousresult of having uniform and balanced interrogation field strength andphasing while at the same time expending substantially less energy forthe production and maintenance of such fields.

In a more particular sense, the present invention provides novelstructural and positioning attributes and relatinships for thefield-producing coils and, further, provides new and improved conceptsfor the configuration and operation of the drive circuits used forexciting the field-generating coils in interrogation or surveillancesystems of the aforementioned type. In accordance with these concepts,coil drive excitation is applied and maintained in a new and improvedmanner involving series-resonant voltage injection circuitry whichapplies a controlled burst which is continuously updated in response toconditions actually prevailing at the field-generating coils. Also, inaccordance herewith, the various field-generating coils within a systemare maintained in continuous balance with one another, so as to enhanceboth efficient and economic operation while simultaneously furtherenhancing detection accuracy and consistency through the maintenance ofbalanced and predictable field density, configuration, and fluxdirection.

The foregoing attributes of the invention and the advantages providedthereby will become increasingly apparent upon further consideration ofthe ensuing specification, particularly when considered in light of theappended drawings, both of which set forth particular embodiments of theinvention to illustrate the underlying concepts thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an enlarged, pictorial perspective view illustrating ageneralized entry and egress passage defined by mutually-spaced "panels"housing the field-generating coils;

FIG. 2 is a perspective representation of a pair of oppositely-spacedfield-generating coils, illustrating the general nature thereof and atypical pattern for currents and magnetic flux;

FIGS. 3a, 3b, and 3c are a succession of schematic representationsdepicting the field-producing coils of a typically surveillance area inaccordance herewith, in which each opposite side has a pair ofvertically-stacked coils, arrows being used to show resultant fluxdirections for various applied excitation phases, as labelled;

FIG. 4 is a side view representation of a pair of field-generating coilsarranged in a first form of mutual disposition, with a space between thecoils;

FIG. 5 is a view similar to FIG. 4 but showing improved coil positioningin accordance herewith;

FIG. 6 is a pictorialized perspective view similar to FIG. 1 but showinga different type of panel structure for the field-generating coils;

FIG. 7 is an enlarged cross-sectional elevational view taken through theplane VII--VII of FIG. 6;

FIG. 8 is an electrical schematic diagram illustrating preferredfield-generating excitation circuitry in accordance herewith;

FIG. 9 is a simplified schematic diagram based upon FIG. 8 but showingalternative and improved features therefor;

FIG. 10 is a further simplified schematic diagram analogous to FIG. 9but showing the general principle involved; and

FIG. 11 is a system block diagram generally illustrating the overallnature of the preferred coil-driving and control circuitry in accordanceherewith.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1 for an understanding of the general operatingenvironment, an example of a typical egress or entrance passage 10 isdepicted as defined by a pair of mutually-spaced "panels" 12 and 14,which constitute vertically-disposed, generally planar bulkhead memberswhich may be on the order of about five feet in height and two feet inwidth, mutually spaced from one another by a distance on the order ofthree feet (by way of very generalized illustrative dimensions). Asshown in FIG. 1, each of the panels 12, 14 may include an upper loopportion and a lower loop portion, giving the panel a somewhat figureeight-shaped appearance which is symmetrical about a vertical axisthrough the center of the panel. Both the upper loop portion and thelower loop portion of each panel house an interrogation field-generatingcoil of the general nature shown at 16 and 18 in FIG. 2, each such coilpreferably having a generally rectangular configuration (e.g., generallysquare) but with rounded corners dictated by the flexibility of theelectrical conductor from which the coils are formed. As shown in FIG.1, both of the panels 12 and 14 rest upon a desired lower supportsurface 20, which may be the floor or walkway. Various means ofsupporting the two panels in their aforementioned relationship may nodoubt be utilized.

A modified form of the field coil-housing panels, designated by thenumerals 12' and 14'; is illustrated in FIG. 6. In a general way, thesepanels are much the same as those illustrated in FIG. 1, but includedifferent appearance facors designed primarily for aesthetics. Thus, thepanels 12' and 14' are truncated pyramids in both side and endelevation, and have six openings rather than two, as depicted in FIG. 1.In such an arrangement, the upper coil (e.g., 16 or 18) encircles thetop four openings and the lower coil (e.g., 116 and 118) encircles thelower four openings; that is, the coils overlap in the middle of thepanel so that they both encircle the two central openings. The form ofpanels 12' and 14' are discussed further hereinafter.

It has been customary in the past to form the field-generating coils 16and 18 from wide strap-like copper conductor (for example, about one andone-half inches wide by one-tenth inch thick), but in accordance withthe present invention it is preferred to use much finer stranded wire(for example, No. 8 or No. 10 stranded copper wire) which mayadvantageously be of the type in which each strand is separatelyinsulated from the others (i.e., "Litz" wire). As explained below, useof stranded conductor provides lower effective resistance while actuallyusing considerably less copper in the coils. When such strandedconductor is used, the multiple-turn coils are preferably "potted" orencapsulated, as by epoxy, so as to form a substantially rigid hoop, asgenerally shown in FIG. 2.

FIG. 2 also illustrates the general flux pattern produced by circulatingcurrents within the coils having the direction shown by the arrows, inaccordance with which flux patterns will be developed surrounding eachof the conductor sections in the generally circular pattern illustrated.With the currents in the two coils travelling in the same direction, aresultant flux vector such as that shown at 22 will be produced, passinglaterally through the center of both coils. With the coils disposed inpanels opposite one another, as illustrated in FIG. 1, it will thus beseen that the resultant flux 22 will be in a horizontal right-to-leftpattern for the current direction shown. Since as stated above the coilsare energized so as to be in electrical oscillation, the alternatingcurrent will thus produce alternating and oppositely-directed fluxvectors, i.e., horizontal and alternating between right-to-left andleft-to-right flux vector orientation.

In order to maximize the likelihood of detecting markers or tags whichmay be physically positioned in any given random orientation, it ispreferred to institute regular changes in the relative direction ofcurrent flow through the various coils constituting a set; thus, wherefour coils are utilized, as generally shown in FIG. 1, three particulardifferent excitation phase relationships or conditions will produce thethree different resultant flux vectors depicted by the arrows shown inFIG. 3.

More particularly, it has previously become known in the art thatdirection of the resultant flux vector (e.g., vector 22 in FIG. 2)produced by a pair of adjacent field-generating coils defining asurveillance area or passage may be changed as a function of therelative phasing of the excitation applied to the respective coils.FIGS. 3a, 3b and 3c illustrate representative such variations in anillustrative surveillance area 10 whose sides are defined by a pair ofvertically-stacked field-generating coils 16, 116 on the left and 18,118 on the right, in the general arrangement illustrated in FIG. 1.Accordingly, in FIG. 3a, a Phase Condition I analogous to thatillustrated in FIG. 2 produces flux vectors in each coil which are alloriented in the same general direction and which thus are aiding orreinforcing in a pair of adjacent coils such as coils 16 and 18. FIG. 3billustrates a Phase Condition II in which the phasing of the coils 18and 118 has been reversed from that of FIG. 3a, and in which theresultant flux vectors thus oppose in each pair of laterally-opposed andoppositely-spaced coils. In FIG. 3, the phasing (Phase Condition III) issuch that the flux vectors in coils 16 and 18 are directed opposite toone another and also opposite to that of their respectivevertically-associated coils 116 and 118, respectively. This produces thevertically-oriented field whose vector is designated by the numeral 122.

The timing, application, and control of the field-generating excitationto produce the different flux patterns depicted in FIG. 3 in acontinuous and rapidly-changing sequence is one major aspect of thepresentation invention, and it is described hereinafter in connectionwith the circuitry depicted in FIG. 8. Prior to addressing that,however, attention is first directed to FIGS. 4 and 5. In FIG. 4, onearrangement is illustrated for a pair of vertically-alignedfield-generating coils (e.g., coils 16, 116), by which a vertical fluxvector may be obtained. FIG. 5 illustrates a new and preferred coildisposition for such a vertically-aligned coil system, in accordancewith another attribute of the present invention.

More particularly, in previous electromagnetic field surveillancesystems, the production of vertically-oriented flux vectors such as thatillustrated in FIG. 3c has not generally been considered, or in anyevent has not been implemented, probably as a result of a belief thatthe two different conditions represented in FIGS. 3a and 3b producesufficient field variation to detect marker tags oriented in anyposition which is reasonably likely to be encountered in real-lifesituations. In actuality, however, it is desirable to have a verticalfield such as that shown in FIG. 3c, in order to provide assurance thatthe system will in fact detect any and all tags, with essentiallyabsolute reliability, and also to further enhance accurate detectionwhile eliminating erroneous alarms, which of course are veryundesirable.

Accordingly, in considering systems for implementing the vertical fluxvector condition in the most desirable way, it has been found that thevertically-aligned spaced coil relationship shown in FIG. 4, whileuseful to produce the vertical vector, noentheless has certainundesirable attributes whereas the unusual partially-overlappedconfiguration shown in FIG. 5 produces very desirable improvements fromthe standpoint of providing increased field uniformity whilesubstantially reducing power requirements for the system. Moreparticularly, the results of comparative evaluations for a typical coilarrangement of the type shown in FIG. 4 (coils vertically aligned butspaced vertically from one another a distance on the order of one andone-half inches), and for the most preferred overlap arrangement inaccordance herewith (for coils on the order of twenty-five inches high,an overlap on the order of five inches per coil, i.e., total overlaparea of ten inches for both coils) is illustrated in the two sets oftables below.

    TABLE 1      CASE I        72 - - - 1                                 1 - -      70 - - 1                                1 -  68 - 1       1  66 1    2       2                 2       2  64 1 2             2             2             2  63 2 3 4   4    3       2       2       3    4      4 3  60 7 8 8 7 6  5 4   3        2   2        3   4 5  6 7 8 8  58 + +     + 9 7 6 5  4  3        2   2        3  4  5 6 8 + + +  56 7 8 8 7 6  5 4       3       2     2       3   4 5  6 7 8 8  54 2 3 4   4    3      2       2      3    4   4 3  52 1 2            2             2            2     50 1    2      2                   2      2  48 - 1                    1  46 - - 1                                   1 -  44 - -     - - 1                               1 - - - Z 42 - - - - - - - - - - - 1           1 - - 1       1 - - - - - - - - - -  40 - - - - - - - - - - - - -     - - - - - - - - - - - - - - - - - - - - - - - - - - A 38 - - - - - - - -     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - X 36 - - -     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -     I 34 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -     - - - - - S 32 - - - - - - - - - - - - - - - - - - - - - - - - - - - - -     - - - - - - - - - -  30 - - - - - - - - - - - - - - - - - - - - - - - -     - - - - - - - - - - - - - - -  28 - - - - - - - - - - - 1       1 - - 1          1 - - - - - - - - - -  26 - - - - 1                               1     - - -  24 - - 1                                   1 -  22 - 1                              1  20 1    2       2                 2       2     18 1 2             2           2             2  16 2 3 4   4    3     2       2       3    4   4 3  14 7 8 8 7 6  5 4   3        2   2     3   4 5  6 7 8 8  12 + + + 9 7 6 5  4  3        2   2        3  4  5 6 8     + + +  10 7 8 8 7 6  5 4   3       2     2       3   4 5  6 7 8 8  8 2 3     4   4    3      2         2      3    4   4 3  6 1 2            2          2            2  4 1    2      2                   2      2  2 - 1                                       1  0 - - - -  - - - -  - - - -  - - -     -  - - - -  - - - -  - - - -  - - - -  - - - -  - - - -  0     5     1 0        1 5    2 0    2 5    3 0    3 5    4 0 4 5 Y AXIS

    TABLE 2      CASE I        72 - - 1                                   1 -      70 - 1                              1  68 - 1      2    2                 2    2       1  66 1   2                                 2  64 1 2  3     3                   3     3  2  62 2 4      4    3                 3    4     3  60 7 9 9 8 7 6 5  4    3               3    4  5 6 7 8 9 8  58 + + +     + 8 7 6 5  4    3             3    4  5 6 7 9 + + +  56 7 9 9 8 7 6 5     4    3             3    4   5 6 7 8 9 8  54 2 4       4    3       3    4       3  52 1 2 3         3                 3         3 2  50 1      2  3      3                   3       3  2  48 1  2   3   3               3   3   2  46 1 2  3      3                     3      3  2     44 2 3 4    4   3      2         2      3   4    4 3 Z 42 7 9 9 8 7 6 5     4   3                 3   4  5 6 7 8 9 8  40 + + + + 8 7 6 5   4   3             3   4   5 6 7 9 + + + A 38 7 9  9 8 7 6  5  4    3           3      4  5  6 7 8 9  8 X 36 3 5 6    6  5   4    3         3    4   5  6    6     4 I 34 3 5 6    6  5   4    3         3    4   5  6    6 4 S 32 7 9  9 8     7 6  5  4    3           3    4  5  6 7 8 9  8  30 + + + + 8 7 6 5   4     3             3   4   5 6 7 9 + + +  28 7 9 9 8 7 6 5  4   3         3   4  5 6 7 8 9 8  26 2 3 4    4   3      2         2      3   4     4 3  24 1 2  3      3                     3      3  2  22 1  2   3   3                         3   3   2  20 1  2  3      3                   3      3  2  18 1 2 3         3                 3         3 2  16 2 4       4       3               3    4       3  14 7 9 9 8 7 6 5   4    3     3    4   5 6 7 8 9 8  12 + + + + 8 7 6 5  4    3             3    4  5 6     7 9 + + +  10 7 9 9 8 7 6 5  4    3              3    4  5 6 7 8 9 8  8     2 4      4    3                3    4      3  6 1 2  3     3               3     3  2  4 1   2                                 2  2 - 1        2    2                 2    2      1  0 - - - -  - - - -  - - - -  -     - - -  - - -  -  - - - -  - - - -  - - - -  - - - -  - - - -  0     5      1 0    1 5    2 0    2 5    3 0    3 5    4 0 4 5 Y AXIS

    TABLE 3      CASE II        72 - - - - - 1                             1 - - - -      70 - - - 1                                    1 - -  68 - - 1             1 -  66 - 1                                     1  64 1   2     2                       2     2  62 1 2  3 3       2                 2         3 3  2  60 5 6 6 5 4   3     2               2     3   4 5 6 6  58 +     + 8 6 5 4  3     2               2     3  4 5 7 + +  56 5 6 6 5 4   3      2               2     3   4 5 6 6  54 1 2  3 3       2     2       3 3  2  52 1   2    2                         2    2  50 - 1                                     1  48 - - 1          1 -  46 - - - - 1                               1 - - -  44 - - - -     - - - - 1                       1 - - - - - - - Z 42 - - - - - - - - - -     - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  40 - -  - - -     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - A 38     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -     - - - X 36 - - - - - - - - - - - - - - - - - - - - - -  - - - - - - - -     - - - - - - - - - I 34 - - - - - - - - - - - - - - - - - - - - - - - - -     - - - - - - - - - - - - - - S 32 - - - - - - - - - - - - - - - - - - - -     - - - - - - - - - - - - - - - - - - -  30 - -  - - - - - - - - - - - - -     - - - - - - - - - - - - - - - - - - - - - - - -  28 - - - - - - - - - -     - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  26 - - - - -     - - - 1                       1 - -  - - - - -  24 - - - - 1                       1 - - -  22 - - 1                                   1     -  20 - 1                                     1  18 1   2     2                  2     2  16 1 2  3 3       2                 2       3 3  2      14 5 6 6 5 4   3     2               2     3   4 5 6 6  12 + + 8 6 5 4     3     2               2     3  4 5 7 + +  10 5 6 6 5 4   3     2           2     3   4 5 6 6  8 1 2  3 3       2                 2       3 3     2  6 1   2    2                         2    2  4 - 1                      1  2 - - 1                                   1 -  0 - -     - -  - - - -  - - - -  - - - -  - - - -  - - - -  - - - -  - - - -  - -     - -  - - - -  0     5     1 0    1 5    2 0    2 5    3 0    3 5    4 0     4 5 Y AXIS

    TABLE 4      CASE II        72 - - - 1                                 1 - -      70 - - 1                                1 -  68 - 1       1  66 1  64 1  2           2             2           2  62 2 3     3             2     2           3     3  60 6 7 7 6 5 4   3        3   4 5 6 7 7  58 + + + 7 6 5 4   3                     3   4 5 6 9 +     +  56 6 7 7 6 5  4   3                     3   4  5 6 7 7  54 2 3      3                             3      3  52 1  2        2  50 1   2              2     2              2  48 1   2           2               2           2  46 1  2          2               2          2     44 1 2 3   3      2                 2      3   3 2 Z 42 6 7 7 6 5 4   3         2           2      3   4 5 6 7 7  40 + + + 7 6 5  4  3        2     2        3  4  5 6 9 + + A 38 6 7 7 6  5  4   3                   3   4     5  6 7 7 X 36 2 3 4     4    3                 3    4     4 3 I 34 2 3 4         4    3                 3    4     4 3 S 32 6 7 7 6  5  4   3               3   4  5  6 7 7  30 + + + 7 6 5  4  3        2     2        3     4  5 6 9 + +  28 6 7 7 6 5 4   3      2           2      3   4 5 6 7 7     26 1 2 3   3      2                 2      3   3 2  24 1  2          2                 2          2  22 1   2           2           2           2     20 1   2              2     2              2  18 1  2                    2  16 2 3      3                         3      3  14 6 7     7 6 5  4   3                     3   4  5 6 7 7  12 + + + 7 6 5 4   3                      3   4 5 6 9 + +  10 6 7 7 6 5 4   3       3   4 5 6 7 7  8 2 3     3           2     2           3     3  6 1  2               2             2           2  4 1  2 - 1                  1  0 - - - -  - - - -  - - - -  - - - -  - - - -  - - - -     - - - -  - - - -  - - - -  - - - -  0     5     1 0    1 5    2 0    2 5        3 0    3 5    4 0 4 5 Y AXIS

In the above tables, Case I shows the vertical-vector field intensitydistribution in a plane passing through the center of the surveillancepassage (e.g., 10) perpendicular to the coil-housing panels (e.g., 12and 14); i.e., the plane YZ, according to the axis shown in dashed linesin FIG. 1. Within Case I, Table 1, pertains to the slightly spaced coilarrangement depicted in FIG. 4, while Table 2 pertains to the overlappedarrangement shown in FIG. 5. Within each such table, the numbers in thefirst column at the left identity vertical distances, in inches, upwardfrom the floor 20 (labeled "Z axis") while the numbers along the bottomidentify distances in inches across the width of the passage (labeled "Yaxis"). The remaining numbers in the tables represent peakvertical-vector field intensity (flux density) in Gauss, at all variousindicated Y-Z coordinate points for all phases of field excitation(i.e., maximum vertical-vector flux density obtained for any phase ofapplied excitation as illustrated in FIG. 3).

Accordingly, Case I, Table 1 above illustrates both graphically andnumerically the magnetic field intensity distribution along the verticalflux axis of the surveillance field for the field-generating coilarrangements shown in FIG. 4, in which a small spacing exists betweenthe two vertically-aligned coils (for example, a spacing on the order offive to six percent of the overall height of two generally rectangularcoils), with applied excitation sufficient to produce a minimum fieldintensity of on the order of one Gauss at the weakest point in the field(actually, many points have somewhat less than that, as indicated by the"-" signs). Under these conditions, as Table 1 illustrates, the resultis a field whose intensities vary over a wide range, in which many ofthe measured field intensities are of a far greater magnitude thannecessary, representing substantial wasted power.

On the other hand, with reference to Case I, Table 2, and under the samebasic conditions described above except for having the coils in theoverlapped configuration illustrated in FIG. 5, it will be seen thatmuch less variation, i.e., much greater field uniformity, is the overallresult, and that the magnitude of both the undesirably low and theunnecessarily high flux intensities are considerably improved over whatis shown in Table 1.

The improvements in field intensity distribution illustrated in Case I,Tables 1 and 2 and described generally above are further evidenced andillustrated in Case II, Tables 3 and 4, which have the same generalbases but depict the resulting field intensities in a plane parallel tothat of Case I but located outward along the X axis a distance of teninches therefrom. As will be seen by inspection, the field intensitydistribution obtained with the overlapped coil geometry of FIG. 5,contained in Table 4 of Case II, is once again greatly superior to thatof the spaced-coil (FIG. 4) arrangement set forth in Table 3.

A most important point to make clear with respect to the data of theabove tables is that the level of field-generating coil excitationrequired to produce a given minimum field intensity at any point as setforth in Tables 2 and 4 is substantially less than that required toproduce the same minimum field in accordance with the results in Tables1 and 3; in fact, the required current is decreased by on the order ofone-third, thus reducing power consumption by on the order of one-half(since power is a function of the square of the current). Of course, itmust also be remembered that use of the preferred stranded conductors toimplement the field coils additionally reduces required excitation, andin particular, losses while also using significantly less copper thanthe wide strap-form conductors used heretofore. These benefits are verysignificant, since they not only reduce expense outright but also reduceambient heat, thus promoting reliability, etc. A highly furthersignificant benefit obtained in this manner from the greater resultingfield uniformity is enhanced detection accuracy and consistency, whichof course is the most important consideration of all, detection accuracymeaning not only increased sensitivity to enable detection of low-levelsignals, but also increased selectivity and repeatability, eliminatingerroneous alarms resulting from marginally low-level detection signalswhich are difficult to distinguish from noise, transient disruptions,etc.

Accordingly, the overlapped coil configuration depicted in FIG. 5represents a very significant system enhancement. With respect to theparticularities of the overlap, it may be stated that various degrees ofoverlap produce degrees of improvement generally representative of theresults depicted in Tables 2 and 4, but an optimal condition of overlapis felt to be on the order of about ten inches total for a pair ofgenerally rectangular (or trapezoidal) coils approximately twenty-threeinches wide and twenty-eight inches high each, i.e, each coil isvertically extended approximately five inches beyond the horizontal lineat which their adjacent boundaries would otherwise be contiguous, acondition which may be generally identified as being on the order ofabout 20-25% overlap. Distinctly favorable results are achieved even atsignificantly greater (as well as less) overlap, up to for example,about 35%, or even somewhat more than that, but less than the point atwhich the net effect becomes more like that of a single coil, as wouldresult from superimposing or only slightly offsetting the two coils, orthe point where the net effect is more like that of two separatevertically-spaced coils, as in FIG. 4.

Further, with respect to the results obtained by the overlapped coilrelationship shown in FIG. 5, in particular the reduction in currentrequirements, and considering the preferred coil winding structureinvolving the use of smaller-diameter stranded wire, the drive currentmay be markedly decreased even though an actual increase in fieldintensity is obtained at the weakest points, and a more uniform fieldintensity obtained. In this regard, the smaller-diameter stranded wireprovides the desirable attribute of substantially lowered totaleffective resistance, because of lower AC resistance (due to less skineffect) at the nominal 10 kHz oscillation frequency for the coils,compared to the physically large strap-type conductors utilized in priorsystems, while actually using less copper. Thus, although the DCresistance of such strap-type conductors was substantially less thanthat of, for example, No. 10 stranded wire, the total resistance underoperating conditions is considerably less with the stranded conductoreven though much less copper is used, taking the decreased AC resistanceinto consideration. Accordingly, the Q of the oscillating circuit mayactually be increased by a significant amount even though the nominal(DC) resistance of the stranded conductor is substantially higher thanthat of the strap conductor utilized previously, and even though someforty turns of the stranded No. 10 conductor are used to replace the sixturns of copper strap utilized in prior systems.

A further enhancement in the overlapped form of the drive coilsillustrated in FIG. 5 is shown in FIG. 7, which may be considered torepresent a sectional view through the panels 12' or 14' of FIG. 6 aswell as the panels 12, 14 of FIG. 1. In essence, the principle involvedis to wind the respective coils in a relatively loosely-bundled mannerand to in effect interleaf the windings of one coil with the other, atleast to the extent of spreading the coils into two or more groupings ofturns and placing the turns of one such coil between the turns of theother. The effect of this is to lower the inductance of each coilwithout changing its ampere-turns, by which the field remainssubstantially the same in overall shape and makeup, but very intenselocalized field-intensities are reduced and, most importantly, therequired energy, and time, to bring the electromagnetic field to a stateof sustained oscillation is also reduced. At the same time, reducing thelevel of excitation required due to the resulting lower inductanceprovides for lower capacitance requirements and enhances moreconservative circuit design.

Referring more particularly to the arrangement shown in FIG. 7, both tothe upper coil 16 and lower coil 116 are shown as being provided in theform of two bundles, designated 16(a) and 16(b), and 116(a), 116(b),respectively, each bundle being securely held in place within the panelby suitable supports 17, 117 attached to the panel structure. As will beunderstood, the two bundles of each coil are connected in series so asto form in effect a single coil. In the arrangement illustrated, all ofthe coil bundles are angled somewhat from vertical so that the lowerbundles may be placed between the diverging lower extremities of theupper bundles in order to form the "overlap" described above. With thisarrangement, each set of coil bundles may be symmetrically and equallyspaced with respect to a vertical plane 113 through the center of thepanel, in which a detection loop ("antenna") may be located. Of course,the arrangement of FIG. 7 should be understood as generally illustratingthe underlying principle, as well as showing one particular preferredembodiment, in that the coil-separating "bundling" and interleavingconcept shown could be carried much further.

Referring now to the preferred drive circuitry 30 illustrated in FIG. 8,it will be noted that such circuit includes four different, butidentically-configured, series-resonant voltage-injection drive circuitbranches, designated 31, 32, 33, and 34, respectively. In each suchseries circuit, there is a capacitor, inductor and an included effectiveresistance, such components being designated by the prefix "C", "L", or"R", followed by a suffix number corresponding to the circuit branch inwhich it is located, e.g., C31, L31, R31, etc. Of these components, theinductance in reality constitutes one of the associated surveillancefield-generating coils, e.g., vertically-stacked coils 16, 116 andadjacent vertically-stacked coils 18, 118, and such inductances are solabelled in FIG. 8 and the resistance R is the effective AC resistance(as described before) of each coil plus the equivalent resistance of thedriver devices. Neglecting for the amount the transformers appearing ineach such circuit branch, which are discussed below, each of theseries-resonant coil-excitation branches 31-34 inclusive additionallyinclude a series-mode square wave generator, comprising (in theembodiment shown) a pair of totem-pole FETs, designated for convenience131, 231 (branch 31); 132, 232 (branch 32), etc. As illustrated, eachsuch pair of FETs is series-connected between a "B+" DC voltage sourceand ground, with their gates connected for alternate or oppositeactuation, by a "driver" input, by which either FET in each such pairmay be oppositely switched on and off, depending on the polarity of theapplied driver signal. For purposes of present discussion, each suchdriver input may be considered as an independent switching controlhaving a frequency or pulse-repetition rate equal to the resonantfrequency of the series coil excitation circuit which it controls,although in fact each such "driver" may constitute a control line from acommon master system microprocessor controller.

In general, the operation of each of the series excitation circuits31-34 inclusive may be in either of two selective modes, i.e.,application of selective opposite-polarity pulse actuation signals bythe associated driver to the interconnected gates of a given pair ofseries-connected FET switches will cause one or the other such switch toconduct, depending upon the polarity of the applied excitation. Eachsuch FET (which may be of the type designated "BUZ 24" devices)(manufactured by Siemens) has an integral back-biased diode; thus, suchselective control signals applied to the gates will cause current flowwithin the series-resonant circuit branch either in a direction towardthe FETs or away from them, toward the ground point, which is shown atthe capacitor end of each branch. This has the effect of selectivelychanging the phase of the coil excitation from one to the other of itspossible conditions, thus controlling the resultant direction of theinduced current in, and flux produced by, that inductor. Accordingly, itwill be seen that by appropriate selection and timing of the applieddriver control signals applied to each of the paired FET switches, thedifferent flux conditions illustrated in FIG. 3 may be produced. By wayof illustration, using the encircled "x" designation illustrated in FIG.8 at one or the other end extremity of each of the field-generatingcoils L31, L32, etc. and with the indicated respective positive-going("high") or negative-going ("low") control signals applied to the FETswitch gates, each of the series-resonant circuit branches will havecurrent flow in the direction indicated by the arrow, and the resultantmagnetic flux condition will be that illustrated in FIG. 3(c), producinga resultant vertical flux vector.

It is to be particularly noted, in connection with the aforementionedcoil driver circuit 30, that use of the series-resonant voltage-sourcetype injection drive circuits 31-34 shown is a distinct departure fromthe coil-excitation circuitry used hereintofore. That is, in priorsystems the resonant circuit was in the form of a parallel "tank"circuit, with the field-generating coil being centertapped and connectedto the supply voltage. In such a circuit, appropriate switching resultedin the selective grounding of one or the other end of each of thecentertapped inductors, thus causing current flow in one or the otherdirection through half of the inductor. Such a circuit is at its verybest only fifty percent efficient even if driven with maximum efficiency(minimum losses). Also, such a circuit operates as having acurrent-source input, or on a constant-current principle, having aseries-connected current-limiting resistor to establish the desiredcurrent level in the circuit. In the case of the illustrated, preferred,series-resonant circuit, the mode of operation is as a voltage source,rather than a current source, and the full length of thefield-generating coil is driven at all times, rather than merelyone-half of it. While discussed more fully hereinafter, the resultingform of the voltage applied to the series-resonant circuits ispreferably of square-wave form, of a nominal frequency of about 10 kHzin the most preferred embodiment (which is the highest which may be usedwithout having to comply with FCC requirements). By using such a squarewave drive, circuit efficiency is further maximized, such that with arelatively high-Q series resonant circuit (on the order of 100, ormore), the voltage and current present within the resonant circuitrapidly build as the square wave is applied, the series resonance of thecircuit at the nominal 10 kHz frequency of excitation presentingessentially zero reactance, and the effective impedance thus beingmerely the included resistive component. The oscillating current thusrapidly builds, but in accordance herewith it should also be noted that(as discussed more fully herebelow), in order to maximize theresponsiveness of the circuit, i.e., to minimize the time required tobring the circuit energy level up, and down from, sustained resonance,it is preferred to both begin and end each different phase condition ofexcitation by applying a substantially greater "boost up" and "boostdown" voltage, and to use a lower level of excitation during the periodsof steady-state resonance at the desired peak flux level.

Commenting somewhat further with respect to the operation of the FETswitches, because such components have extremely low resistance whenconducting in the forward direction when turned "On" by their gate(generally considered zero resistance) but additionally have very highresistance in the "Off" state to block unwanted conduction, thetotempole arrangement of paired FETs results in what is in effect azero-impedance switch for applying the field-coil excitation square wavevoltage. Accordingly, there is virtually no power loss, in effect, byusing such switches. In view of the above comments with respect to theadvantages of using series-resonant circuitry, it will therefore beappreciated that each of the series excitation circuits, 31, etc., is anextremely low-loss circuit, with virtually no leakage current throughthe FETs when they are off. Thus, if the Q of the circuit is very high(say, 200), the actual voltage excursion which can be produced withinsuch a circuit from an applied excitation of about twenty volts is onthe order of 2,000 volts, peak-to-peak, at which condition approximately10 amps of current is circulating through, but not being lost in, theseries-resonant circuit. Thus, effiencies approaching 100% can beachieved, whereas, as stated above, the maximum efficiency with priorart excitation circuits was 50%, and the realizable efficiency was muchless than that. Accordingly, the totem-pole (series-connected) FETswitch arrangement noted above is certainly included in the mostpreferred embodiments of the invention; however, it should also be notedthat other functionally similar switching may be utilized, for example,transformers or relays, e.g., reed relays. Also, excitation could beaccomplished by sine wave or the like, but the noted square waveexcitation is inherently more likely to yield greater efficiency.

In view of the foregoing information, it will be appreciated that underconditions of actual operation, each of the four series coil-excitationcircuits 31-34 inclusive undergoes repeated changes in the phase of itsapplied excitation, in order to produce the changes in resultant fluxvector direction noted. Preferably, this is accomplished by applying theaforementioned square wave excitation synchronously with the cycles ofthe AC line voltage prevailing at the place of operation, since in thismanner a high degree of synchronism will automatically be obtained forthe different drives and fields of each installation and, moreover, fordifferent separate installations in areas of proximity, which mightotherwise interact with one another in various ways, i.e., through theelectromagnetic fields produced. A preferred such timing for the appliedsquare wave, which is nominally of 10 kHz frequency, is to set systemtiming on the basis of the AC line voltage frequency and phasing, as bydividing the time of each half-cycle of the 60 Hertz line into threeuniform time segments, and then to synchronize the phasing of the(nominally) 10 kHz square wave driver signal for each of the threedesired different phase (flux vector) conditions to the three successivesuch time segments. Thus, the repetition rate for the different burstsof applied nominal 10 kHz square wave drive is, for three phaseconditions and a 60 Hz line, 180 Hz. In particular, considering the timerequired for building up to full resonance in each phase condition andthe time required for quieting the resonance oscillations of that phasein preparation for the next ensuing phase condition, a preferred totaltime basis for applying each successive different phase condition isabout five and two-thirds milliseconds, during which time there will beon the order of fifty-five cycles of the nominal 10 kHz square wave.Accordingly, each instance of Phase Condition I will be timed tocommence at the zero crossing of the line frequency and continue for120° of the line alternation, whereupon, each Phase Condition II willcommence and continue for a like duration, followed by Phase ConditionIII, etc. This will be true at each different installation, and thuseach surveillance system in a multisystem location will, although beingindependent from the others in all other ways, be synchronized togethernonetheless.

Referring once again to the circuit shown in FIG. 8, it should be notedthat the various coil-excitation circuit branches 31-34 inclusive, inwhich the oscillating currents at resonance characterize the oscillatingelectromagnetic field conditions produced by the associated inductor(coil), are themselves interconnected by coupling transformersdesignated T1-T6 inclusive, transformers T1 and T2 being coupled betweendrive circuit branches 31 and 32, coupling transformers T3 and T4 beingcoupled between drive circuit branches 33 and 34, and couplingtransformers T5 and T6 being coupled between drive circuit branches 31and 33. Each such coupling transformer may be a trifilar-wound toroid,for example having sixteen turns of No. 17 wire and providing a couplingfactor on the order of 0.9999, with inductance on the order of about 2millihenries. As illustrated, the primary winding of each such couplingtransformer is connected into the series coil-excitation circuit, whilethe secondary and tertiary windings are connected in separateoppositely-phased loops controlled (opened or closed) by back-to-backpairs of FETs which are designated (in the loop 36 between circuitbranches 31 and 32) 40, 41 and 44, 45; in the loop 37 interconnectingcircuit branches 31 and 33, 48, 49 and 52, 53; and in the loop 38interconnecting circuit branches 33 and 34, 56, 57, and 58, 59. Asillustrated in FIG. 8, the junction of each of the paired FETs isconnected to ground, while the gate electrode of each such pair isconnected together and coupled to a switch drive whose equivalentcircuits are shown separately and designated by the numerals 60, 62, and64, of which drive 60 and drive 62 are coupled together and controlledat a logic input designated "B", while switch drive 62 has its inputcontrolled from a complementary logic input labeled "A".

The coupling coil system just described is provided for the purpose ofmaking current and phase conditions the same in all four of thefield-excitation coils 16, 18 and 116, 118 and in their respectiveassociated drive circuits 31-34 inclusive. That is, it is very desirableto have the same circulating current level and also the same phasing, orat least the same zero-crossing point (i.e., either directly in phase,or directly out of phase, depending upon the particular phase conditioninvolved) which parameters, in accordance herewith, are sometimesreferred to by use of the expression "current flow conditions". Inessence, the primary windings of each of the coupling transformerscomprise current-sense coils, which reflect and which may control thephase, frequency and magnitude of the circulating current present intheir particular coil-excitation circuit branch. Thus, with due regardfor the winding direction of the transformer coils involved (asillustrated by the dots at the various ends of the couplingtransformers) the secondary and tertiary windings are connected intooperation in specified sequences coordinated with the field coil driveexcitation timing and phasing. This is accomplished by appropriategating signals applied to the control FETs 40, 41, etc., and during theinterval each of the particular circuits involved is actively coupledinto operation, it is in effect made part of a single composite currenttransformer. Since, on an instantaneous basis, current into a currenttransformer must equal current out of it, either directly in phase ordirectly out of phase, the various currents flowing in the respectivewindings of the different coupling transformers are forced to equalize,due to the voltages developed across the windings, and the phases (thatis, zero-crossings and maxima) of the circulating currents are alsoforced into balance, i.e., "current flow characteristics" are broughtinto direct synchronization.

Therefore, the overall effect of the coupling transformers T1-T6 is toforce all of the field-generating coils in a system to operate in directsynchronism with one another, and also to operate at identical currentlevels. In this manner, the inherent slight mismatches between theinductances in the various drive circuit branches, or between thecapacitances in them, or whatever such mismatches or disturbances as mayexist in the actual implementation and operation of a system, areequalized and compensated. In cases where no such mismatches exist, novoltage differences will exist, and no energy will be transferred, fromone such drive circuit branch to the other through the couplingtransformers, but in reality at least some miniscule such mismatches orimbalances are likely to inherently be present, or to occur duringsystem operation due to spontaneous environmental conditions. Whereverany such mismatch occurs, there will be energy transfer from one drivecircuit branch to another, to the extent required to equalize circuitoperation (current) in each. Of course, the desired goal is to have eachof the circuit branches very closely matched in any event, so thatlittle or no energy is transferred between them through the couplingtransformer system. Whatever the situation may be, it will beappreciated that the coupling transformer circuits typify the concept ofa comprehesive correction or compensating means for detecting mismatchand imbalance in the field coil drive circuits and automaticallycorrecting the same, to achieve balanced coil drive circuits byautomatically correcting the same, and thereby achieve steadily-balancedcoil drive operation on a continuous basis. In so doing, the individualcoupling circuits may either inject energy into or take energy out of agiven coil drive circuit, so as to balance it with the others by ineffect applying difference signals between any imbalanced coils. In thisconnection, it should be noted that the concept involved could beimplemented in other ways, for example by using sensing coils wound overthe field-drive coils to provide the circuit access. Of course, switchedinputs and outputs could also be utilized, as could other types ofbalancing networks.

As indicated above, all of the switching required to appropriately openand close the various coupling transformer loops in a mannercorresponding to the phasing illustrated in FIGS. 3(a), 3(b), and 3(c)is accomplished by supplying logic switching inputs to terminals A andB. In FIG. 8, the phasing and transformer winding sense indicated by thedots at one end or the other of the various transformer coils, togetherwith the logic state of the input supplied to the various switching FETs131, 231, etc., will produce the current flow directions indicated bythe arrows in each of the coil-excitation circuit branches and in eachof the coupling transformer loops. Upon due consideration, this may beseen as producing Phase Condition III, illustrated in FIG. 3(c). Withrespect to driver phasing, or logic, it will be noted that each of thedrive sources indicated as being applied to the gates of the FETs in thevarious coil excitation loop branches may also be the same as thoseapplied to the logic inputs A and B, or may at least be controlled froma common driver. Additionally, the "B+" applied to each of the coils ina given system may be taken from a common source, and that source mayalso be controlled as to output level by a system controller. In themost preferred embodiment, each panel may have its own power supply aswell as its own timer or controller, for all of the coils in that panel,and each such power supply should be controllable, by the controller forthat panel. In turn, the various panel controllers should be under thecontrol of a master controller (microprocessor).

A further point of explanation with respect to the particular systemdepicted for purposes of illustration in the Figures herein is that thetwo-panel embodiment illustrated is merely one of many possibleconfigurations, i.e., a system with two passages for egress or entry maybe implemented by using three adjacent panels, three passages would havefour adjacent panels, etc. In each case, the coupling andinterconnection from one panel to the next would be essentially the sameas that illustrated, i.e., all of the different field-generation coilsin all of the different panels would be coupled together for uniformoperation, in the same general manner as that discussed above. Moreparticularly, FIG. 8 illustrates the interconnections for two panels,each having upper and lower coils coupled together in the mannerdescribed, with the upper coils also being intercoupled, for uniformoperation of all four of the coils in both panels. In an analogousthree-panel system, the two coils of the third panel would be coupledtogether in the same general manner as the two coils of either of thepanels illustrated in FIG. 8, and one or the other of the coils in thethird panel would be coupled back to one or the other of the coils inthe second panel, by use of the same type of coupling transformercircuitry. In order to enhance uniform circuit configuration, apreferred such coupling would be to utilize the lower suchfield-generation coils on the first panel and couple it to the uppercoil on the adjacent panel. That is, by adding the primary winding of anadditional coupling transformer in series within drive circuit branch34, the latter would then have two coupling transformer primary windingsjust as is the case in both circuit branches 31 and 33. That additionalcoupling transformer would then be coupled in the same manner over tothe drive circuit branch for the upper field coil in the adjacent panel.Accordingly, a multiple-panel system containing practically anyrealistically desirable number of passages for egress or ingress wouldin effect comprise a cascaded chain of the individual coils, in effectcoupled one to the other in a long series chain. In such a cascadedchain, all of the field generation coils would then have currents ofessentially equal magnituded, whose zero-crossings were essentiallysimultaneous.

A further component percent in the circuit of FIG. 8 is a current-sensetransformer 17, shown coupled into the drive circuit branch 32 forpurposes of illustration. In an actual sense, such a component could becoupled into any of the drive circuit branches, or for that matter intoall of the drive circuit branches. In a particular example, currentsense transformer T7 may be a single-turn primary winding which isactually connected in series in drive circuit branch 32, with asecondary winding having on the order of 200 turns shunted by a resistorR7 connected to ground, thus providing a current sense output terminaldesignated by the numeral 70. The current sense transformer T7 is infact a current transformer, whose purpose is to act as a detecting coilor pickup, to provide a signal whose amplitude is proportional to theamplitude of the circulating current present in drive circuit branch 32.Due to the action of the coupling transformers T1-T6 inclusive, thedetected level of the current in circuit branch 32 will also beindicative of the circulating current in each of the resonant drivecircuit branches 31, 32, 33, and 34.

The current sense output thus provided on terminal 70 is utilized fortwo purposes; first, it provides a direct measure of the level of thecirculating currents in the series resonant drive circuits, and thus maybe used as a feedback signal to control the level of the "B+" suppliesfeeding each of the resonant series circuits. In this manner, regulateddrive currents for controlled steady-state oscillation may be achieved,and it is also possible to attain system resonance in a much more rapidmanner than would normally be true. That is, the B+ supply voltage usedat start-up for each new phase condition may be set at a much higher"boost" level than that of the normal, steady-state supply; alternately,a supplemental higher-output source may be used, switching the same intothe out of operation at the desired times. The latter arrangement isillustrated in FIG. 8, in which the supplemental source "V-Boost" isshown as being applied additively with the normal "B+", through switcheslabeled "S_(B) ", which are also preferably power FETs, configuredsimilarly to the B+FETs 131, 231, etc. In a particular example of theboost voltage, with a steady-state B+ supply level at about twentyvolts, the boost voltage at start-up may instead be on the order of 120volts, applied only for the interval necessary to bring the circulatingcurrents up to the full oscillatory level, which may be determined bymonitoring the output from transformer T7 appearing on terminal 70.While a tapering, continuously-controlled wide-range B+ supply couldthus be used, due to the capabilities provided by the feedbackcircuitry, it is found that in fact by starting with a boost voltage ofthe 120 volt magnitude just indicated, full resonance will be rapidlyachieved due to the high-Q of the circuit, and the supply may simply bedropped to a lower sustaining level after a few cycles (e.g., five) onan open loop basis.

A rapid quenching or quieting of the field coil oscillations at the endof each successive phase condition is also very desirable, in order tomaximize the period of steady-state oscillation. The supplementalswitched source arrangement noted above and shown in FIG. 8 hasparticular advantage in this regard, since this not only allows use of arelatively simple boost supply but also promotes rapid quenching offield coil oscillation while also providing desirable efficiencyenhancements. That is, in order to achieve rapid quenching, it is onlynecessary to open the power FETs controlling both the steady-state B+and the boost source V-Boost. In so doing, not only is thepositive-polarity voltage from the source disconnected, but in additionthe inherent back-biased diodes in the FETs in effect create an inversesquare wave by using the natural circulating currents flowing in thecoils. Thus, circuit oscillations are driven down very rapidly, inessentially the same short time as it takes to achieve boostedoscillation in the first place. At the same time, the drive-down energyis actually routed back to the sources (i.e., both the B+ and V-Boostsources) to further enhance circuit efficiency. Rapid quenching is thusachieved without actually creating or separately applying an inversesquare wave drive-down signal, and the circulating currents in the coilsare utilized productively while their energy is also returned to thesupply sources. This is accomplished merely through use of normalyreverse-biased diodes in the lines from the sources, which areinherently present where the preferred FETs are used as switches.

The second purpose for the output 70 from current sense transformer T7is analogous to that just stated, but has application to the actualphase condition present in the series resonant circuit. Thus, thezero-crossings indicated by the current sense output are indicative ofthose present in the series resonant circuit, and by using this signalas a phase condition feedback the indicated phase may be compared withthe actual phase of the FET driver signals generating the square wavedrive excitation. In the event of error, i.e., phase differencesindicating drive at other than exact resonance, suitable frequencycorrection may readily be made.

More particularly with respect to the power supply level control andphase-correction, or frequency-correction, as referred to above, ageneralized illustration of a preferred implementation is shown in FIG.11. Referring to the latter, the current sense output from terminal 70of transformer T7 is shown coupled back through a peak-detector 72 andphase comparator 74, and through an A/D converter 76, to a systemcontroller (e.g., microprocessor) 78 which is used to produce controlsignals for drive amplitude as well as frequency, that is, the amplitudeof the drive excitation applied to the drive circuit branches, and thefrequency of the switching signals applied to the FETs which "chop" thatapplied excitation. Microprocessor control of the amplitude of the B+supply (designated 80 in FIG. 11) in conjunction with the high-level"boost" start-up (and/or "drive-down") drive procedure described abovewill be readily understood. That is, a "boost voltage" source 82, havinga predetermined high leve, may be placed directly under microprocessorcontrol, to be switched on and off, (and/or even reversed in polarityfor faster drive down, if desired), by the microprocessor (actingthrough a D/A converter 79) at whatever times and for whatever periodsthe microprocessor may be programmed, e.g., the first and/or lasthalf-dozen pulses of the coil drive excitation during each new phasecondition (the timing and duration of which are all controlled by themicroprocessor in any event). When switched on, the boost voltage fromsource 82 may merely override the normal steady-state excitation fromthe variable source 80, which should be applied through a diode which isreverse-biased closed by the higher voltage to drive up the energy levelof the resonant circuit very quickly. Upon completion of thepreprogrammed number of cycles of boost drive, the boost supply 82 ismerely switched off by the microprocessor, leaving the source 80 toforward-bias the aforementioned diode and drive the field coilssufficiently to maintain steady-state oscillation. It is desirable tocontrol source 80 during such steady-state conditions, by using thecurrent-sense output 70 as a feedback signal to the microprocessor, sothat the latter may vary the output of source 80 in accordance withrequirements indicated by the level of the circulating currents sensedand outputted on line 70. One good way of doing this is bypeak-detecting the sensed current on line 70; hence, the peak-detector72 shown. Of course, this procedure could also be utilized tocontinuously control a widely-variable power supply, from a high initial"boost" value to (and during) the lower "run" value.

It is contemplated that the driver frequency applied to the power FETsbe generated by use of a voltage-controlled oscillator, indicated inFIG. 11 at 84, controlled by the microprocessor 78 through a D/Aconverter 86 as a function of the current sense output 70. In thismanner, the frequency of the drive pulses applied to each of the seriesresonant circuits will always be maintained at the actual frequency ofcircuit resonance. That is, the current sense output 70 may be comparedto the phase of the drive frequency being applied to the drive circuits,as by the "phase comparator" 74, and an error signal representative ofany detected difference in the respective zero-crossings may be inputtedto the microprocessor 78 via the A/D converter 76, to cause anappropriate correction by the control applied to the V.C.O. 84. Infurther view of the operation of the coupling coils, discussed above, itwill be appreciated that the resulting operation is exceedingly stable,as well as exceedingly accurate; thus little correction to either thelevel or the phase of the applied drive is likely to be necessary as ageneral matter. Imbalances and diruptions caused by spontaneous ambientconditions may of course occur, but these are also quickly damped outdue to the coupling effects. As an end result, detection accuracy issubstantially enhanced.

It should be noted that, as a still further refinement in theapplication of the square wave excitation to the coil drive circuits,the aforementioned microprocessor controller 78 may desirably beprogrammed to re-set the output frequency from V.C.O. 84 at the start ofeach successive new phase condition of coil excitation so that it startsthat phase at the particular frequency required for oscillation in thatparticular phase condition (bearing in mind that the difference mutualinductances encountered in each such different situation will change theresultant resonant frequency). This may be accomplished by using acount, or word, retained in memory, whose value identifies either theanticipated or the actual frequency of oscillation for the next ensuingphase condition. For example, upon completing a Phase Condition I andupon commencing the next succeeding Phase Condition II, themicroprocessor should have available a digital signal in memorycorresponding to the ancitipated drive frequency to be applied to thefield coils during steady-state resonant operation in Phase ConditionII. By using such a signal as the basis for setting the actual squarewave frequency to be applied to the drive circuits during the nextensuing Phase Condition II, it is much more likely that such drivesignal will from the very beginning be very close to that needed toproduce steady-stated resonant oscillation during the Phase Condition IIabout to commence. Of course, the same is true with respect to eachsuccessive change in phase condition, i.e., a different signal is storedin memory for each different phase condition, each to be used as a basisfor initial setting of the drive frequency to be applied at the start ofeach change in phase condition. These stored command signals may bepredetermined on the basis of actual measured oscillation frequencies,or the stored values could be continuously updated using the actualfeedback controlled V.C.O. set signal from the previous cycle ofoperation in that phase condition. In this manner, much greaterconsistency is achieved with much less feedback-controlled changerequired, all of which in the end substantially enhances detection andanalysis accuracy.

An additional enhancement for the drive circuitry of FIG. 8, inaccordance with the most preferred embodiments of the invention, isillustrated in FIGS. 9 and 10, both of which show simplified versions ofthe overall circuit illustrated in FIG. 8. Referring first to FIG. 9, itis to be noted that the capacitors in corresponding pairs of theseries-resonant drive circuits 31-33 and 32-34, respectively areconnected together at which was previously their grounding terminal, andthe resulting common point is connected to ground through an additionalcapacitor, designated C35 and C36, respectively. The inherentinterconnection of all ground points is indicated by the dashed linedesignated "G". The principle involved in this arrangement isillustrated in FIG. 10, which is basically the same as FIG. 9 but whichutilizes an additional, independent grounding capacitor C35(a), C35(b),and C36(a), C36(b) instead of the commonly-shared capacitors C35 and C36of FIG. 9. In the circuit of FIG. 10, the additional, independentcapacitors just described are each shunted by a switch S₁, S₂, S₃, andS₄, each of which should be understood as being remotely controllableand which, although drawn in a simple, generic style, should of coursebe understood as being any state-of-the-art switch.

Briefly stated, the purpose of the circuit variations depicted in FIGS.9 and 10 is to help offset the shift in the resonant frequency of thedifferent drive circuit branches as a result of the different mutualinductance effects occurring during the different drive phase conditions(hence, the "M" coupling indicated between each pair of field coils).This is accomplished in the indicated manner due to the fact that theadditional capacitors are actually in operation only during one of thetwo possible current-flow directions, namely, when either of the twooppositely-aligned coils (e.g., coils 16 and 18, or coils 116 and 118)are driven in phase with one another. That is, when two such coils aredriven out of phase there will be equal and opposite currents throughthe series-resonant capacitors and to ground in one instance while fromground in the other instance. With the ground side of both suchcapacitors connected together, as illustrated in FIG. 9, the circuitpoint at their junction will thus have a net current flow of zero underthe condition of excitation; therefore, if an additional capacitor isconnected from that point to ground, such capacitor will not have anyactive operation in the circuit performance. On the other hand, when thetwo such currents are in phase, the additional capacitor will have aneffect, since the net current flow at the circuit point in question isno longer zero, but is in effect twice the magnitude of each of the twoequal currents in the opposite circuit branches.

Accordingly, the effect will be to add capacitance to each drive circuitwhere the two energizing currents are in phase, without adding suchcapacitance to the circuits when the two currents are out of phase. Thenet result is a difference in the effective resonant frequency of thecircuits for the two different excitation conditions, such differencebeing of a nature which reduces the frequency shift which otherwisewould be present strictly as a result of the different mutualinductances created as a result of the different excitation phaseconditions. Thus, the oscillation frequencies of the field coils aremade to be more nearly the same in each of the different phaseconditions.

The desired result noted above may also be achieved in other ways, andthe switching circuit arrangement shown in FIG. 10 is illustrative ofone such way. In such a circuit, the various switches S₁ -S₄ inclusivewill, when closed, short out the additional capacitance; thus, bylogical control of the switches, the same effect as noted above may beobtained. That is, with the same logical switching used to produce thediffering coil drive phase conditions and coupling transformerconnections described above, the switches S₁ -S₄ may similarly becontrolled to in effect add and subtract capacitance as desired indifferent phase conditions. Basically, the higher of the resonantfrequencies for the different conditions of coil excitation phasing willremain the same, while the lower frequencies caused by the differentmutual inductances created will be raised toward the nominal higherfrequency level. This is a very desirable effect, notwithstanding thecompensating and balancing measures discussed above, including thecoupling transformer circuits as well as the currentfeedback-microprocessor control arrangement described. In this manner,with more nearly uniform circuit and coil operation in all of thedifferent phase conditions, there will be less power consumption in theform of losses experienced, and perhaps even more importantly, thedetection and analysis which is the end result of the surveillancesystem will be significantly enhanced. One fundamental suchconsideration is the basic fact that the higher the frequency ofresonance, the more cycles of field perturbation sample will beavailable for analysis per unit of real time. Additionally, it is verybeneficial to maintain sampling and analysis at a more continuous,constant repetition rate, which may be better understood and furtherappreciated by reference to copending application Ser. No. 673,015, towhich reference is made.

It is to be understood that the above is merely a description of apreferred embodiment of the invention and that various changes,alterations and variations may be made without departing from theunderlying concepts and broader aspects of the invention as set forth inthe appended claims, which are to be interpreted in accordance with theestablished principles of patent law, including the doctrine ofequivalents.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of applyingcontrolled electrical excitation to the field-generating coil windingsin an alternating electromagnetic field-type surveillance system,comprising the steps: generating an alternating electromagneticsurveillance field across a surveillance passage by using at least apair of inductance coils; applying electrical excitation to said coilsby coupling a power supply means thereto through an electrical drivecircuit for each of said coils; monitoring at least one of the currentflow characteristics constituting circulating current maxima and phasingpresent in at least one of said drive circuits and producing a controlsignal representative thereof; and variably controlling the electricalexcitation applied to at least selected ones of said coils in responseto and as a function of said control signal so as to conform the actualcurrent flow in such selected coils to a substantially uniformcharacteristic.
 2. The method of claim 1, wherein said representativecontrol signal comprises a feedback signal coupled back to said powersupply means, and including the step of using said control signal tovariably control the output of said power supply means.
 3. The method ofclaim 2, wherein said electrical excitation applied to said coilsincludes pulsations, and including the step of controlling said coilexcitation by controlling said power supply means to change the positionwith respect to time of the pulsations in the coil excitation providedby said power supply means.
 4. The method of claim 3, wherein said powersupply means is controlled by changing the frequency of a pulsatingoutput produced thereby.
 5. The method of claim 2, including the step ofusing said feedback signal to variably control said power supply bychanging the magnitude of the output supplied thereby.
 6. The method ofclaim 5, wherein said step of variably controlling said power supplyoutput magnitude is carried out in a manner to decrease the magnitude ofthe pulsating electrical coil excitation from a high level start-upmagnitude to a substantially lower level sustaining magnitude.
 7. Themethod of claim 5, wherein said feedback signal is used to variablycontrol said power supply means by shifting the position with respect totime of the pulsations comprising said coil excitation.
 8. The method ofclaim 1, wherein said steps of monitoring said current flowcharacteristics and producing a signal representative thereof includeslinking at least said selected ones of said field-generating coilstogether and substantially equalizing the circulating currents flowingin each.
 9. The method of claim 8, including the steps of using at leasta pair of field-generation coils on at least one side of said passage toproduce a composite surveillance field therein, linking said coilstogether, and conforming the circulating currents flowing in each. 10.The method of claim 9, including the steps of using at least a pair offield-generation coils on both opposite sides of a surveillance passageto produce a composite surveillance field therein, and effectivelylinking all of said coils together to conform the circulating currentsflowing in each.
 11. Apparatus for applying drive excitation to thealternating-field-generating coils of an electromagnetic field-typesurveillance system, comprising in combination: source means forsupplying electrical drive excitation for energizing said coils togenerate an electromagnetic interrogation field; a separate andindividual series-type electrical drive circuit for each of said coils,each such circuit connecting one such field-generating coil in serieswith at least one capacitance; each of said series-type drive circuitscomprising a series-resonant LC circuit having capacitance andinductance, including the inductance of the field-generating coilconnected in that circuit, whose relative values produce seriesresonance at a frequency which generally corresponds to the desiredfrequency of alternation for the interrogation field; and a plurality ofseparately-controllable switch means, including at least one such switchmeans for separately connecting each of said series-type drive circuitsto said source, such that said interrogation field is produced by eachof said coils under separately-controlled excitation from said source.12. Apparatus as defined in claim 11, wherein each of said switch meansfor separately connecting each of said series-type drive circuits tosaid source comprises means for changing the effective direction of thecurrent flow through its corresponding series-type drive circuit withrespect to the effective direction of current flow in other such drivecircuits.
 13. Apparatus as defined in claim 12, and including a switchactuator coupled to each of said switch means and providing a varyingcommand thereto for controlling the switching operation thereof. 14.Apparatus as defined in claim 13, wherein said switch actuator comprisesa variably-controllable signal-generating device, and afeedback-responsive closed-loop control apparatus for sensingcirculating currents in at least one of said coils and controlling theoperation of said signal-generating device as a function of the sensedcoil current.
 15. Apparatus as defined in claim 14, wherein said controlapparatus detects coil oscillation frequency and controls the outputfrequency of said signal-generating device as a function of detectedcoil oscillation frequency.
 16. Apparatus as defined in claim 12, andincluding controllable means for varying the magnitude of the excitationfrom said source which is applied to said drive circuits.
 17. Apparatusas defined in claim 16, including controllable means for separatelyvarying the magnitude of the excitation from said source which isapplied to each of said drive circuits.
 18. Apparatus as defined inclaim 16, and including a switch actuator coupled to said switch meansand providing a varying command thereto for actuating the switchingoperation thereof.
 19. Apparatus as defined in claim 18, wherein saidswitch actuator comprises a variably-controllable signal-generatingdevice, and a feedback-responsive closed-loop control apparatus forsensing circulating currents in at least one of said coils andcontrolling the operation of said signal-generating device as a functionof the sensed coil current.
 20. Apparatus as defined in claim 19,wherein said control apparatus detects coil oscillation frequency andcontrols the output frequency of said signal-generating device as afunction of detected coil oscillation frequency.
 21. Apparatus asdefined in claim 11, wherein predetermined ones of said series drivecircuits are connected together at a common node located at a pointwhich leaves the said inductance and capacitance of each such circuitconnected together in series, and including an additional capacitanceconnected between said common node and the circuit ground, for shiftingthe resonant frequency of the interconnected drive circuits tocompensate for mutual inductance effects due to particular phaseconditions of the applied excitation.
 22. Apparatus as defined in claim11, wherein at least certain of said drive circuits include at least onesecondary capacitive element and circuit means for operatively switchingsuch element into and out of conduction in a manner shifting theeffective resonant frequency of such drive circuits to compensate formutual inductance effects due to particular phase conditions of theapplied excitation.
 23. Apparatus as defined in claim 12, wherein saidseparately-controllable switch means comprises a power FET device. 24.Apparatus as defined in claim 23, wherein each of said switch meanscomprises a pair of FET devices connected together in seriesback-to-back configuration between said source and a circuit groundreference point, said series drive circuit being connected to thejunction of said two FET devices.
 25. Apparatus as defined in claim 24,wherein said switch actuator comprises a variably-controllablesignal-generating device coupled to the control gate of said FET devicesto switch such devices on and off.
 26. Apparatus as defined in claim 25,including a feedback-responsive closed-loop control apparatus forsensing coil oscillation frequency and controlling the output frequencyof said signal-generating device as a function of the sensed coiloscillation frequency.
 27. Apparatus as defined in claim 11, andincluding means for monitoring the frequency of electrical oscillationin at least one of said coils and means for synchronizing pulsations inthe drive excitation applied to such coil with the monitored oscillationfrequency in such coil.
 28. Apparatus as defined in claim 27, andincluding means for sensing the magnitude of the oscillating currentflow in said coil, and means for varying the magnitude of the driveexcitation pulsations applied to the drive circuit for such coil inresponse to the sensed oscillation current magnitude.
 29. Apparatus asdefined in claim 27, wherein said drive excitation pulsations comprisespulses having a generally rectangular wave shape, and including meansfor maintaining the width of said drive pulses substantially the same asthe half-cycle time of the monitored coil oscillation frequency. 30.Apparatus as defined in claim 29, and including means for sensing themagnitude of the oscillating current flow in said coil, and means forvarying the magnitude of the drive excitation pulsations applied to thedrive circuit for such coil in response to the sensed oscillationcurrent magnitude.
 31. Apparatus as defined in claim 29, wherein saidsynchronizing means operates to set the occurrence of said driveexcitation pulses at the occurrence of at least certain of saidhalf-cycles of coil oscillation, to thereby place such pulses andhalf-cycles substantially in time coincidence with one another. 32.Apparatus as defined in claim 31, and including means for sensing themagnitude of the oscillating current flow in said coil, and means forvarying the magnitude of the drive pulsations applied to the drivecircuit for such coil in response to the sensed oscillation currentmagnitude.
 33. Apparatus as defined in claim 31, including means formaintaining the timing of said drive pulsations substantiallysynchronized with the monitored coil oscillation frequency, such thatindividual drive pulsations are applied substantially simultaneouslywith half-cycles of coil oscillation.
 34. Apparatus as defined in claim33, and including means for sensing the magnitude of the oscillatingcurrent flow in said coil, and means for varying the magnitude of thedrive pulsations applied to the drive circuit for such coil in responseto the sensed oscillation current magnitude.
 35. Apparatus forgenerating an alternating electromagnetic interrogation field in an areasurveillance system, comprising in combination: at least a pair ofinductance coils spaced opposite from one another across a surveillancearea; a source of electrical excitation for driving each of said coilsin field-generating oscillation; a plurality of individual drivecircuits, each such circuit separately coupling said source to adifferent one of said coils, for separately applying said excitation tosaid coils; means for monitoring current-flow conditions in at leastcertain of said coils during field-generating oscillation and forproviding a control signal representative of predetermined suchcurrent-flow conditions; and means for varying the coil-drivingexcitation applied to at least certain of said coils as a function ofsaid control signal, to thereby closely coordinate and balance thefield-generating operation of the different coils on the basis of saidpredetermined current-flow conditions.
 36. The apparatus of claim 35,and including synchronous switching means for periodically changing theeffective direction of current flow in at least selected ones of saidfield-generating coils from the direction previously existing therein toan opposite direction to thereby change the composition of the magneticflux in the field, and for applying pulsations of drive excitation tothe coils receiving changed current-flow direction in synchronism withpulsations of drive excitation applied to other coils.
 37. The apparatusof claim 36, wherein said synchronous switching means is arranged tomaintain the occurrences of the drive excitation pulsations which areapplied to coils with changed current-flow direction substantially inunison with the occurrences of the drive excitation pulsations which areapplied to other coils.
 38. The apparatus of claim 37, and includingmeans for coupling said source of coil-driving excitation to an a.c.line supply, and means for synchronizing the switching operation of saidswitching means with the frequency of alternation of the applied a.c.line.
 39. The apparatus of claim 38, wherein said switching means isadapted to make said periodic changes in coil current-flow direction insynchronism with the timing of individual cycles of alternation of theapplied a.c. line.
 40. The apparatus of claim 35, wherein said sourceprovides pulsating coil-driving excitation, and including means formonitoring the frequency of oscillation of at least certain of saiddrive circuits during coil-driving operation thereof and means forsynchronizing the pulsation rate of the coil-driving excitation fromsaid source with the monitored oscillation frequency of said certaindrive circuits.
 41. Apparatus as defined in claim 40, and includingmeans for varying the amplitude of said drive excitation in relation tosaid coil drive circuit current flow conditions as a function of saidcontrol signal.
 42. Apparatus as defined in claim 40, wherein said driveexcitation pulsations comprise pulses having a generally rectangularwave shape, and including means for maintaining the width of said pulsessubstantially the same as the half-cycle time of the monitored coildrive circuit oscillation frequency.
 43. Apparatus as defined in claim42, and including means for varying the amplitude of said driveexcitation pulses in relation to said coil drive circuit current flowconditions as a function of said control signal.
 44. Apparatus asdefined in claim 42, wherein said synchronizing means sets theoccurrence of said generally rectangular pulses in accordance with theoccurrence of at least certain of said half-cycles of coil drive circuitoscillation, to thereby place such pulses and half-cycles in timedrelation with one another.
 45. Apparatus as defined in claim 44, andincluding means for varying the amplitude of said drive excitationpulses in relation to sensed coil drive circuit current amplitude. 46.Apparatus as defined in claim 44, including means for maintaining therepetition rate of said generally rectangular pulses substantially thesame as the monitored coil oscillation frequency, such that said pulsesare applied to said coils substantially in unison with half-cycles ofthe oscillating current flow in the coils.
 47. Apparatus as defined inclaim 46, including means for sensing coil oscillation circulatingcurrent magnitude, and means for varying the magnitude of the appliedcoil drive excitation in relation to the sensed circulating currentmagnitude.
 48. The apparatus of claim 35, wherein said means formonitoring current-flow conditions in at least certain of said drivecircuits during their coil-driving operation comprises at least oneadditional coil coupled to at least one of said drive circuits.
 49. Theapparatus of claim 48, wherein said additional coil comprises at leastone transformer winding which is electrically connected into said drivecircuit to receive at least a portion of the current flow presenttherein.
 50. The apparatus of claim 49, wherein said transformercomprises a coupling transformer, and further including at least oneadditional coupling transformer having a winding electrically connectedinto another of said drive circuits, each of said coupling transformershaving at least one additional winding and including circuit meansinterconnecting said additional windings to thereby interconnect saidcoupling transformers and thus couple said drive circuits together forbalanced operation of their respective field-generating coils.
 51. Theapparatus of claim 50, wherein said means for monitoring current-flowconditions further includes at least one current transformer having aprimary winding connected into one of said drive circuits and asecondary winding connected into a feedback circuit to provide afeedback signal which is a function of current flow in said one drivecircuit.
 52. The apparatus of claim 51, and including means forreceiving said feedback signal and for varying the magnitude of thepulsating excitation applied to the field-generating coils of thosedrive circuits of which said feedback signal is representative, as afunction of said feedback signal.
 53. The apparatus of claim 51, andincluding means for receiving said feedback signal and for varying thetiming of the pulsating excitation applied to the field-generating coilsof the drive circuits of which said feedback signal is representative,as a function of said feedback signal.
 54. The apparatus of claim 50,and including means for receiving said feedback signal and for varyingthe frequency of the pulsating excitation applied to thosefield-generating coils of the drive circuits of which said feedbacksignal is representative, as a function of said feedback signal.
 55. Theapparatus of claim 54, and including means for receiving said feedbacksignal and for varying the magnitude of the pulsating excitation appliedto the field-generating coils of those drive circuits of which saidfeedback signal is representative, as a function of said feedbacksignal.
 56. The apparatus of claim 48, and including synchronousswitching means for periodically changing the effective directions ofcurrent flow in at least selected ones of said field-generating coilsfrom the direction previously existing therein to an opposite direction,to thereby change the composition of the magnetic flux in the field;said switching means maintaining the pulsations of drive excitationapplied to those coils receiving changed current flow direction insynchronism with the pulsations of drive excitation applied to othercoils.
 57. The apparatus of claim 56, and including means for couplingsaid source of coil-driving excitation to an a.c. line supply, and meansfor synchronizing the switching operation of said switching means withthe frequency of alternation of the applied a.c. line, and wherein saidswitching means is adapted to implement said periodic changes in coilcurrent flow direction in synchronism with the timing of individualcycles of alternation of the applied a.c. line.
 58. The apparatus ofclaim 57, wherein said coil comprises at least one transformer windingand is electrically connected into said drive circuit to receive atleast a portion of the current flow present therein, and wherein saidtransformer comprises a coupling transformer, and further including atleast one additional coupling transformer having a winding electricallyconnected into another of said drive circuits, each of said couplingtransformers having at least one additional winding and includingcircuit means interconnecting said additional windings to therebyinterconnect said coupling transformers and thus couple said drivecircuits together for balanced operation of their respectivefield-generating coils.
 59. The apparatus of claim 57, wherein saidmeans for monitoring current-flow conditions further includes at leastone current transformer having a primary winding connected into one ofsaid drive circuits and a secondary winding connected into a feedbackcircuit to provide a feedback signal which is a function of current flowin said one drive circuit.
 60. A method of energizing thefield-generating coils in an area-surveillance system of the type whichuses spaced oppositely-disposed such coils to generate an alternatingelectromagnetic surveillance field in the area between such coils, saidmethod comprising the steps of using a separate individual drive circuitto separately energize each such field-generating coil and drive thesame into oscillatory electrical conduction, correlating the operationof said separate drive circuits so that each functions to drive itsassociated field-generating coil conjointly with the driving of theother such coils, generally in accordance with a predetermined mutualfield-generating relationship; and balancing the electromagnetic fieldcomponents produced by the respective coils by transferring energy fromone of said drive circuits to another thereof.
 61. The method accordingto claim 60, wherein said step of balancing electromagnetic fieldcomponents is carried out by linking one such drive circuit to anotherthrough interconnecting circuitry distinct from excitation supplycircuitry.
 62. The method according to claim 61, wherein said step ofbalancing electromagnetic field components is carried out by sensing thecirculating currents flowing in each such drive circuit and implementingcorresponding increases and decreases therein to conform such currentsinto a predetermined mutual relationship.
 63. The method according toclaim 62, wherein said steps of sensing currents and implementingcorresponding increases and decreases is carried out by using couplingtransformer means interconnecting the corresponding drive circuits.